Amenorrhea is more common in trained female athletes than in their less active counterparts. It has been suggested that amenorrhea is an adaptation to conserve energy in trained females whose energy intake is marginally adequate. This study aims to determine daily energy expenditure and its components in amenorrheic and eumenorrhic trained females using whole-room indirect calorimetry. Both diet and activity are controlled before measurements of energy expenditure to ensure that these variables do not affect energy expenditure measures. The energy expended during and following a typical training run is also determined using a treadmill and metabolic cart system. Fuel oxidation during all measurements of energy expenditure is compared between groups. Eumenorrheic subjects have the same measurements made in both the follicular and luteal phases of their menstrual cycle. It is hypothesized that amenorrheic athletes will show a greater energetic efficiency over 24 hours and during a typical training session. This has implications for the control of body energy flux in amenorrhea and how this is mediated via the gonadotrophic hormones.